The invention relates to a method for preventing vibrations in a multi-nip calender, and to an arrangement for preventing vibrations in a multi-nip calender.
The multi-nip calender examined in this invention comprises an upper roll and a lower roll, which are equipped with variable-crown means within the rolls. The upper and/or lower roll can be loaded by external hydraulic cylinders in a direction parallel to plane of the set of rolls, thus generating the desired overall nip pressure and nip pressure distribution in the roll nips of the set of rolls. Two or more intermediate rolls are disposed between the upper and lower roll, in alignment with these. Support arms are attached to bearing houses provided at the ends of the intermediate rolls, the intermediate rolls being articulated from the support arms to the calender frame. Load-relieving means, such as hydraulic cylinders, are connected to the support arms for relieving the weight of the intermediate rolls proper and that of associated auxiliary devices, such as doctor blades, steamer boxes and output rolls.
In a multi-nip calender, there are frequently generated vibrations impairing the calendering result, particularly in cases where the calendering conditions (including nip pressure, moisture of the fiber web, speed of the fiber web) have remained constant over a long period. DE patent specification 10036574 discloses a method for calendering a fiber web in a multi-nip calender with a view to preventing such vibrations generated in a multi-nip set of rolls. The method alters the nip load exerted on a multi-nip calender. Such altered nip load may prevent vibrations in a set of rolls, yet involving the problem of poorer printing characteristics of certain paper grades.